A while back I posted some simplified DF underwater fighting rules.
I've since revised them a little bit. Here is what I have and intend to use next time the PCs play "will swim for gold."
These are basically the same, except that I've made it explicit that Aquatic creatures get certain bonuses. I also changed the damage on attacks into/out of the water to half damage, which is just easier than a per-die minus that doesn't impress guys doing 2d+8. There are a few other small changes, as well.
DF Simplified Underwater Combat
- Swimming move is per Basic Set p. B354 / Exploits, p. 21.
- Weapon skills are capped by Swimming skill, unless you are under the effects of the Swim spell on or are Aquatic or Amphibious.
- No Reach 1+ weapon swinging attacks underwater. Sawing, close combat weapon swings, etc. use thrust-based damage. Limitations are based on maximum reach.
- Thrusting and unarmed attacks suffer no additional penalties.
- No thrown ranged attacks under water (including Missile spells); special underwater ranged weapons exist and use special stats.
- Attacks into and out of the water are at -4, 1/10 range, do half damage, some attacks can't penetrate water at all. Based on attack coming from one environment to the other.
- Shield bashes do half damage and are -2 x DB to hit
- No Parry except unarmed or purely close combat weapons, no Block (but you get DB), Dodge is normal for your move, no Retreat (except with Ethereal Body or Walk Through Water or Aquatic creatures).
Plus the usual rulings as needed. I've already made a fair amount of concessions to "if it looks like a Bond-vs-Frogmen fight, it's not penalized" so I'm not inclined to make more. I'd deal out of a lot of "No, even though you're a Weapon Master with the Swim spell on." You wouldn't need to make a lot of Swimming rolls, or anything else besides "fight more or less normally."